Certain drive systems are subject to torsional stresses which are stored as reactive torque in a drive train. When drive power to the system is interrupted, the reactive torque is released as backspin and, if an uncontrolled release of torque occurs, personal injury and/or property damage can result. For example, deep well submersible pumps such as progressing cavity pumps driven by sucker rod strings are commonly used to pump oil from deep wells. The drive strings for these submersible pumps usually have a relatively small diameter of ¾ to 1⅛ inches. Such drive strings are commonly used in wells that vary from 1,500′ to 6,000′ in depth, 3,000′ being a common average.
Progressing cavity pumps include a stator, which is attached to a production tubing at the bottom of a well, and a rotor which is attached to a bottom end of the drive string. Due to the rotational resistance of the pump and the weight of the fluid being pumped, the drive sting is torsionally deformed. Progressing cavity pumps are frequently used to pump viscous crude oil, which is often laden with sand or other impurities. As a result, the elongated drive string is subject to considerable torsional force. This torsional force is stored in the elongated drive string as reactive torque. In a 3,000 foot string, as many as several hundreds of revolutions of torsion can be stored in the string if viscous sand laden crude oil is being pumped. The deeper the well and the heavier the liquid being pumped, the larger the torsional force. Upon release, the larger the torsional force, the faster the backspin. Excessive backspin speeds will occur unless a backspin braking system is used to maintain the backspin speed below a safe limit while absorbing and dissipating the energy. The safe speed is determined by the speed rating of the drive head, the power transmission system, or the prime mover.
Commonly, pulleys and belts are used to transmit power from the prime mover to the drive head. If pulleys rotate fast enough, such as during uncontrolled backspin, they will shatter due to tensile stresses in the rim resulting from centrifugal forces. Fragments from shattered sheaves are very dangerous to operating personnel. This is particularly true if an electric motor is used as a power source because such motors offer almost no resistance to reverse rotation.
Brakes which simply prevent the release of reactive torque stored in the drive string are unsatisfactory for two reasons. First, it is preferable that in the case of an electric motor drive, the motor be able to restart unattended when power is restored. In order to ensure a successful unattended restart, the motor must start without load. If the reactive torque in the drive string is not released prior to restart, the motor may not be capable of restarting and the motor may be damaged as a result. Second, if pump repair or replacement is required any unreleased torque in the drive string can be extremely dangerous for unaware workmen. Severe personal injury can result from the unintentional release of reactive torque in such drive strings.
Consequently, braking systems have been developed in an effort to prevent overspeed rotation of the shaft. Centrifugal as well as fluid brake systems are known for backspin control.
Fluid brake systems include a pump engaged only during backspin, which pump circulates hydraulic fluid or lubricating oil from a reservoir to a bearing case through a restricted orifice or valve. The resistance of the fluid created by the restriction serves to control the release of reactive torque. In other fluid brake systems, the circulated fluid is used only during backspin to operate a disc brake mounted on the shaft (see U.S. Pat. No. 5,358,036 by Mills). Fluid brake systems have the disadvantage that the stored energy dissipated by the brake heats the fluid and, thus, may break down the fluid, damage seals and degenerate the lubricating quality of the fluid, which may damage bearings and gears in the pump or the brake system. Of course, leakage of the fluid may lead to catastrophic failure of the system.
U.S. Pat. No. 4,797,075 by Edwards et al. describes a centrifugal brake system including a plurality of circumferentially distributed and leaf spring mounted brake shoes. The centrifugal force acting on the brake shoes overcomes the resetting force of the leaf spring at excessive rotation speeds. However, the brake is not unidirectional and fatigue in the leaf springs may lead to the brake being at least partially engaged even during forward rotation. Moreover, very cold temperatures may lead to excess stiffness of the leaf springs and consequently excessive speeds of the shaft. Other centrifugal brake systems are disclosed in U.S. No. 4,216,848 of Toyohisa Shiomdaira, and U.S. Pat. No. 4,993,276 of Edwards.
U.S. Pat. No. 6,079,489 by Edwards discloses another type of centrifugal brake mechanism which acts on a brake housing to provide a backspin retarder. The housing serves as a stationary brake member. The mechanism has weighted movable brake members, which are spring biased toward an inner inactive or disabled position, and which, during forward rotation of the drive shaft, are mechanically locked in the inner position. During reverse rotation of the drive shaft, the brake members are unlocked and permitted to move radially outwardly under the influence of the centrifugal force to engage with the brake housing. In addition, cams are provided for urging the movable brake members during reverse rotation into more intimate contact with the brake housing.
U.S. Pat. No. 6,079,489 by Hult et al. and US2008/0296011 disclose further centrifugal brake systems wherein the brake shoes are spring biased towards a disengaged position and moved to an active, braking position by the centrifugal force and movable cams. In this type of centrifugal braking system, the biasing of the movable brake members toward the inner disabled position reduces the maximum braking force achievable and requires the use of the additional cam members. However, the use of additional mechanical parts may increase manufacturing costs and increase the chance of mechanical failure. Also, the use of cams may lead to the brake locking up which is undesirable whenever the stored reactive torque in a shaft is to be completely released. Thus, an improved centrifugal brake system, more particularly a backspin braking system is desired which overcomes at least one of the disadvantages of prior art systems and can preferably be incorporated into the drive head of a progressing cavity pump.